The present invention relates to a signal amplifier and an optical signal receiver using the same, and more particularly, a signal amplifier suitable for amplification of a burst signal and a burst optical signal receiver using the same.
In recent years, there has been increased expectation of a high-speed burst optical transmission system such as a PON (passive optical system) for use in an optical subscriber system.
In FIG. 1, a conceptual diagram of the PON system is shown. A plurality of subscribers #1-#n are connected to a coupler 200 through optical transmission lines 200-1-200-n. Further, coupler 200 and an switching office 201 are interconnected through an optical backbone line 202.
A cell-formed optical signal in a burst form is output from each of the plurality of subscribers #1-#n. The optical signals 203-205 are forwarded to switching office 201 through coupler 200 and optical backbone line 202.
Here, as shown in FIG. 1, the distance between coupler 200 and each of the plurality of subscribers #1-#n is different, resulting in different levels of cell-formed optical signals being output from coupler 200. This necessitates an optical signal receiver on the switching office to have a wide input dynamic range so as to amplify these signals in common.
In FIG. 2, there is shown an exemplary configuration of the optical signal receiver for receiving the optical burst signals under the aforementioned condition, which was formerly proposed by the inventor of the present invention. In FIG. 2, an optical signal is received in a photodiode (PD) 100, to convert into an electric current signal. The signal is then input to a pre-amplifier 101.
Pre-amplifier 101 converts this electric current signal into a voltage signal. A trans-impedance amplifier 103 constituting pre-amplifier 101 includes an amplifier 103A having a diode 103C connected in parallel with a feedback resistor 103B, so as to produce a wider dynamic range.
When an excessive input signal is received, diode 103C is turned on to reduce the feedback resistance so as to prevent the amplifier from saturation. Thus, desirable output waveform having a wide input dynamic range can be obtained from a buffer amplifier 104 incorporating a signal polarity inverting function.
Signal amplifier 102 is constituted by a master-slave automatic threshold control (ATC) circuit 106 and a limiter amplifier 108. Signal amplifier 102 amplifies a weak signal being output from buffer amplifier 104 provided in pre-amplifier 101 to obtain logic signals of sufficient level.
Master-slave ATC circuit 106 includes a master peak detector 106B for detecting the maximum input signal value and a slave bottom detector 106A for detecting a relatively minimum value from the peak detection level. These outputs are resistively divided by a serially connected resistive voltage divider 106C. Thus an intermediate value is obtained as a DC voltage level for setting a threshold level against a limiter amplifier 108.
Now, in each subscriber #1-#n, optical signal is emitted by driving a laser diode. Here, bias current is made to flow from a few bits before the cell-formed optical signal. This reduces light emission delay and improves output waveform.
In FIG. 3, there is shown a diagram of input current amplitude versus output voltage amplitude in pre-amplifier 101 in the optical signal receiver shown in FIG. 2. An extinction ratio on transmitter in driving the laser is, for example, on the order of 10 dB or less. Input current amplitude I includes DC level II, as a bias current, corresponding to the extinction ratio mentioned above.
Meanwhile, as shown in the figure, the characteristic of input current amplitude versus output voltage amplitude in pre-amplifier 101 becomes nonlinear because of diode 103C. This produces a large amount of ascent on the xe2x80x980xe2x80x99 level in the output III of pre-amplifier 101.
As a result, a problem arises that the amplitude level to be detected by bottom detector 106A becomes higher than the transient minimum value. To cope with this problem, master-slave ATC circuit 106 is applied in signal amplifier 102, as shown in FIG. 2. This enables to detect the signal amplitude level certainly, because the use of slave bottom detector 106A enables to detect the minimum value after the peak level is determined.
However, the signal amplifier shown in FIG. 2, which has formerly been proposed by the inventors of the present invention, is used for one-way signal transmission. There is a problem that the signal is possibly deteriorated caused by external noise, etc.
Namely, when an external noise enters in the transmission line between pre-amplifier 101 and signal amplifier 102, there arises a drift in the input signal. In such a case, the output signal of limiter amplifier 108 is drifted because the threshold value produced from voltage divider 106C responds slowly and, as a result, little variation is produced. This produces difficulty in normal transmission.
FIG. 4 shows signal waveform responses {circle around (1)} to {circle around (6)} at the corresponding positions of signal amplifier 102 shown in FIG. 2. In FIG. 4A, the peak value and the bottom value of the input signal waveform response {circle around (1)} are detected by peak detector 106B and bottom detector 106A, respectively.
The example shown in FIG. 4A illustrates a case that an external noise enters at the part of input signal waveform response {circle around (1)} shown by the circle with the broken line. In FIG. 4B, there is shown an example that the intermediate value between the peak value and the bottom value is set as a threshold value {circle around (4)} by voltage divider 106C.
Further, FIG. 4 illustrates that a normal phase output {circle around (5)} and an inverse phase output {circle around (6)} are obtained as the output signals. These outputs are derived from the input signal waveform response {circle around (1)} being referenced from the threshold value {circle around (4)}. It is to be understood that noise component included in the output remains unchanged.
As shown in FIG. 4C, according to the configuration of the optical receiver shown in FIG. 2, there is a problem that waveform deterioration is produced by external noise, etc., because of single signal transmission.
Accordingly, it is an object of the present invention to provide a signal amplifier configuration and an optical signal receiver using the same. This can solve the problem of waveform deterioration produced by external noise, etc. in a signal amplifier of the formerly invented optical signal receiver shown in FIG. 2.
According to the present invention, an optical signal receiver to solve the aforementioned problem includes; a first master-slave level detector for detecting a direct voltage level of a normal phase signal; and a second master-slave level detector for detecting a direct voltage level of an inverse phase signal. Differential signal transmission is realized by adding respective signals to the signal components alternately. Thus, according to the present invention, differential transmission becomes possible using two master-slave level detectors constituted by mutually symmetric configuration. It becomes possible to obtain a signal amplifier which can cope with various transient responses produced at the top of a burst cell, and to protect against the disturbance produced by external noise.
As one aspect of the present invention, there is provided a signal amplifier including; a first level detector for detecting a direct voltage level of a normal phase signal; a first adder for adding an inverse phase signal to the detection output of the first level detector; a second level detector for detecting a direct voltage level of the inverse phase signal; a second adder for adding the normal phase signal to the detection output of the second level detector; and a differential amplifier for differential-amplifying the outputs of the aforementioned first and second adders.
As another aspect of the present invention, a signal amplifier includes; a first level detector for detecting a direct voltage level of either a normal phase signal or an inverse phase signal; a first adder for adding either the inverse phase signal or the normal phase signal to the detection output of the first level detector; a second adder for adding the normal phase signal to the inverse phase signal; and a differential amplifier for differential-amplifying the outputs of the aforementioned first and second adders.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.